2009_Book_FoodChemistry

436 8 Food Additives
Table 8.3. Taste threshold values of some alicyclic sulfamic
acids (Na-salts)
c tsw
c tsw
R (mmol/l) R (mmol/l)
Cyclobutyl 100 Cycloheptyl 0.5–0.7
Cyclopentyl 2–4 Cyclooctyl 0.5–0.8
Cyclohexyl 1–3
8.8.4 Monellin
The pulp of Dioscoreophyllum cumminsii fruit
contains monellin, a sweet protein with a molecular
weight of 11.5 kdal. It consists of two peptide
chains, A and B, which are not covalently
bound. Their amino acid sequences are shown in
Table 8.4.
The conformation is known (Fig. 8.7 and 8.8).
As a result of cross reactions with an antiserum
against thaumatin (cf. 8.8.5), sequence
Y(13)ASD in a β-turn is regarded as the site
of contact with the sweetness receptor. It corresponds
to sequence Y(57)FD of thaumatin.
The separated individual chains are not sweet.
When the chains are recombined, a sweet taste
is restored slowly, but the sweetness intensity
of the original native protein is not reached.
This strongly suggests that peptide chain separation
results in irreversible conformational
changes. However, combination of synthesized A
and B chains gave a product with the same
sweetening strength as natural monellin. The
thermal stability of the protein was increased
Fig. 8.7. A two dimensional representation of the conformation
of the peptide chains of monellin (a) and
thaumatin (b). (β-structure: →| ; α-helix: ∂ ; β-turn: ⊃;
N, C, or N A ,N B ,C A ,C B : N and C termini of the chains)
(according to Kim et al., 1991)
by covalently bonding the two peptide chains
via the amino acid residues A2 and B50 (cf.
Fig. 8.9). For this purpose, a synthetic gene
Table 8.4. Amino acid sequences of the A and B chains of monellin. The sequence YASD shown in bold type,
which is localized in a β-turn, is regarded as a part of the structure responsible for the cross reaction of monellin
with antibodies against thaumatin as well as for making contact with the sweetness receptor (cf. Table 8.5 and
Fig. 8.7 and 8.8)
5 10 15 20
A-chain: F a R E I K G Y E Y Q L Y V Y A S D K L F R
A D I S E D Y K T R G R K L L R F N G P
V P P P
5 10 15 20
B-chain T b G E W E I I D I G P F T Q N L G K F A V
D E E N K I G Q Y G R L T F N K V I R P
C M K K T I Y E N E
a Ca. 10% of the A chains also contain phenylalanine at the N-terminal (Phe-A-chain).
b Ca. 19% of the B chains also contain threonine at the N-terminal (Thr-B-chain) and N-terminal glycine is absent
in ca. 24% (de-gly 1 -B-chain).